1. Field of the Invention
The present invention is related to a method for keeping track of accumulated stress damage and, more particularly, to a method for accumulating stress damage caused by surface effect temperature differentials in the rotor of a high pressure steam turbine.
2. Description of the Related Art
As is well known, when an object is heated or cooled unevenly, stress can be formed due to the expansion or contraction of part of the object relative to another part of the object which is fixed in place. The stress induced by such temperature differential can be calculated by known techniques, as described in ASME Paper No. 63-PWR-16, "Prevention of Cyclic Thermal-Stress Cracking in Steam Turbine Rotors," by W. R. Berry, published by the American Society of Mechanical Engineers in 1964. Steam turbines are commonly monitored to detect the operating temperature so that surface effect temperature differentials which induce stress in the rotor of the turbine can be calculated. The resulting information is used, for among other purposes, to estimate stress damage to the rotor 8, illustrated in FIG. 3, by temperature changes during the operation of the turbine.
The estimated stress damage may be accumulated by incrementing a mechanical counter by an amount corresponding to the amount of stress damage induced during a single period of substantially continuous heating or cooling. Recently, non-volatile storage devices have been used in place of mechanical counters. However, regardless of whether the storage device is a mechanical counter or a non-volatile storage device, the accumulated stress damage counter may fail. For this reason, duplicate counters or storage devices are usually provided for redundancy. However, when considering that the life of a turbine is typically 30 years, even double or triple redundancy may be insufficient and each extra device increases the cost. If, despite such precautions, the devices fail, the failure may go unnoticed, and in the case of non-volatile counters, the accumulated stress damage prior to failure may be completely lost.